EDTA titrations effect

Cu, Ni, Co, Cr, Fe, or Al, even in traces, must be absent when conducting a direct titration of the other metals listed above if the metal ion to be titrated does not react with the cyanide ion or with triethanolamine, these substances can be used as masking reagents. It has been stated that the addition of 0.5-1 mL of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with solochrome black and also with xylenol orange (see below). 

The greater the effective formation constant, the sharper is the EDTA titration curve. Addition of auxiliary complexing agents, which compete with EDTA for the metal ion and thereby limit the sharpness of the titration curve, is often necessary to keep the metal in solution. Calculations for a solution containing EDTA and an auxiliary complexing agent utilize the conditional formation constant K" = aM aY4- Kt, where aM is the fraction of free metal ion not complexed by the auxiliary ligand. 

Another major step in many analyses is separation (Chapters 22 to 25). When, because of the method chosen or the nature of the sample, this unit operation is not required, much effort can be saved. For example, if a masking agent will complex an interfering metal ion in an EDTA titration, a separation step may be avoided. Where a separation is essential, a choice among several techniques is usually available. In general, separation involves the formation of two phases, physically separated, one containing the material of interest and the other the interference. Either phase may be a gas, liquid, or solid. Thus six major types of separation processes are possible. Once separation has been effected, the quantitative determination by physical means is often straightforward. 

Usually Am ,y > m..jy- and hence the couple becomes a stronger reducing agent in the presence of excess EDTA. The effect is pronounced with the cobalt(III)-cobalt(II) couple, whose potential is shifted about 1.2 V, so that cobalt(II) can be titrated with cerium(IV). Alternatively, cobalt(III) can be titrated to cobalt(II) with chromium (II) as the reducing agent. ... 

D-5 The Effect of Other Complexing Agents on EDTA Titration Curves... 

A quantitative description of the effects of an auxiliary complexing reagent can be derived by a procedure similar to that used to determine the influence of pH on EDTA titration curves. Here, a quantity am is defined that is analogous to 0 4. 

The success of EDTA titration depends on the high Kf values for most metal ions (Table 9-2). Since the ligand is taken to be a rather strong base, the pH will be important in determining its effective concentration. The [Y "] is given by aoy times the EDTA not in MY, the metal ion complex. 

Note the analogy of this result with that outlined for the effect of solubility product in precipitation titrations discussed above. A great many compounds have been proposed as indicators for metal ions in EDTA titrations. These species are generally organic compounds that form colored chelates with metal ions in a range of pM that is characteristic of the cation and dye. One example is Eriochrome black T, which is blue at pH 7 and red when complexed with a variety of metal ions. 

EDTA titrations are still widely used because of their great versatility with respect to the analysis of a large number of different metal cations. Furthermore, the technique can be made more selective by adjusting the pH or by the use of compounds that effectively remove interfering cations from the titration (masking agent). The method is inexpensive and reasonably accurate. 

We determined the cation ratio by EDTA titration using Xylenol Orange as the indicator. The pH was buffered to the 5.0-6.5 range using sodium acetate/acetic acid to optimize indicator effectiveness. Each crystal was weighed and dissolved in dilute HCl from which two aliquots were taken. One was treated directly with EDTA to obtain the total metal concentration according to the reaction... 

In this experiment the concentrations of Ga + and Mg + in aqueous solutions are determined by titrating with EDTA. The titration is followed spectrophotometrically by measuring the absorbance of a visual indicator. The effect of changing the indicator, the pH at which the titration is carried out, and the relative concentrations of Ga + and Mg + are also investigated. 

The magnesium will be liberated quantitatively and may then be titrated with a standard EDTA solution. Where mixtures of metal ions are analysed, the masking procedures already discussed can be utilized or the pH effect exploited. A mixture containing bismuth, cadmium and calcium might be analysed by first titrating the bismuth at pH = 1-2 followed by the titration of cadmium at an adjusted pH = 4 and finally calcium at pH = 8. Titrations of this complexity would be most conveniently carried out potentiometrically using the mercury pool electrode. 

Epoxy-based membrane of 2-[(4-chloro-phenylimino)-methyl]-phenol reveals a far Nemstian slope of 43 mV per decade for Pb+2 over a wide concentration range CIO 6 to 10 1 mol dm-3). The response time of the electrode is quite low (< 10 sec) and could be used for a period of 2 months with a good reproducibility. The proposed electrode reveals very high selectivity for Pb(II) in the presence of transition metal ions such as Cu2+, Ni2+, Cr and Cd2+at concentrations l.()xl() 3 M and 1.0><10 4 M. Effect of internal solution concentration was also studied. The proposed sensor can be used in the pH range of 2.50 - 9.0. It was used as an indicator electrode in the potentiometric titration of Pb+2 ion against EDTA. 

After equilibrium, filtration, and pH adjustment, the residual calcium ion concentration was then titrated by EDTA solution. A higher residual calcium ion concentration indicates better inhibition activity and, therefore, more effectiveness in controlling calcium carbonate deposition in the treated water. As shown in Table III, at dosages of 1 to 5 ppm, the polyacrylic acid was more effective than the acrylic acid/N-(hydroxyalkyl)-acrylamide copolymers. 

The complexometric method for determination of Ca(II) and Mg(II) is based on two titrations with EDTA in alkaline solution, one where both ions are determined together and the second after one of them has been masked with a specific complexing agent. The effect of interfering heavy metals such as Cu, Fe, Mn or Zn can be avoided by adding cyanide. The AOAC Official Method 964.01 for determination of acid-soluble... 

You can see from the example that a metal-EDTA complex becomes less stable at lower pH. For a titration reaction to be effective, it must go to completion (say, 99.9%), which means that the equilibrium constant is large—the analyte and titrant are essentially completely reacted at the equivalence point. Figure 12-9 shows how pH affects the titration of Ca2+ with EDTA. Below pH 8, the end point is not sharp enough to allow accurate determination. The conditional formation constant for CaY2" is just too small for complete reaction at low pH. 

Formation constants for EDTA are expressed in terms of [Y4-], even though there are six protonated forms of EDTA. Because the fraction (aY4 1 of free EDTA in the form Y4 depends on pH, we define a conditional (or effective) formation constant as K = aYj Kf = MY" 4 /[M"+ [EDTA], This constant describes the hypothetical reaction Mn+ + EDTA MY 1-4, where EDTA refers to all forms of EDTA not bound to metal ion. Titration calculations fall into three categories. When excess unreacted M"+ is present, pM is calculated directly from pM = — log M l+]. When excess EDTA is present, we know both [MY"-4] and [EDTA], so IM"+] can be calculated from the conditional formation constant. At the equivalence point, the... 

EDTA (ethylenediaminetetraacetic acid) (H02CCH2)2NCH2CH2N-(CH2C02H)2, the most widely used reagent for complexometric titrations. It forms 1 1 complexes with virtually all cations with a charge of 2 or more, effective formation constant Equilibrium constant for formation of a complex under a particular stated set of conditions, such as pH, ionic strength, and concentration of auxiliary complexing species. Also called conditional formation constant. 

Figure 13 Colour changes in the titration of Ca2+ with EDTA using murexide as indicator. The effect of pH on pCa at...

A colorimetric method based on the inhibitory effect of EDTA on the Mn(II) catalyzed oxidation of malachite green by periodate was reported [32]. An alternative method based on using Fe(III) instead of Mn(II) was proposed [33]. The reduction of the absorbance of Bi(III) bromo-pyrogallol red tenside ternary complex upon the addition of EDTA has been exploited for its determination. Calibration curves obtained at 650 nm were linear over the range of 0.2-6 pg/mL of EDTA [34]. EDTA in ophthalmic solutions could be assayed by spectrophotometric titration using Mg(II) as the titrant and Arsenazo I as the indicator. The working range was 0.05-2 pg/mL [35]. 

Assay Dissolve 250 mg of sample, accurately weighed, in 100 mL of water and 4 mL of 2.7 N hydrochloric acid, boil to effect solution, and cool. While stirring, preferably with a magnetic stirrer, add about 30 mL of 0.05 M disodium EDTA from a 50-mL buret, then add 25 mL of 1 N sodium hydroxide and 300 mg of hydroxy naphthol blue indicator, and continue the titration to a blue endpoint. Each milliliter of 0.05 M disodium EDTA is equivalent to 6.807 mg of CaS04. Fluoride Determine as directed under Fluoride Limit Test, Appendix IIIB, using 1.67 g of sample, accurately weighed. Lead Determine as directed in the APDC Extraction Method under Lead Limit Test, Appendix IIIB. 

A simple and rapid method for the iodometric determination of microgram amounts of chromium(ni) in organic chelates is based on the oxidation of chromium(III) with periodate at pH 3.2, removal of the umeacted periodate by masking with molybdate and subsequent iodometric determination of the liberated iodate . Iodometric titration was also used for determination of the effective isoascorbate (see 2) concentration in fermentation processes . The content of calcium ascorbate can be determined with high sensitivity by complexometric titration with edta, which is superior to iodometry. The purity of /3 -diketonate complexes of Al, Ga, In and Ni was determined by complexometric titration with edta at pH 5.5-3, with RSD < 0.01 for determining 5-30% metal ion. Good analytical results were obtained by a similar procedure for the metal content of 15 lanthanide organic complexes. ... 

Rather than simply measuring the pH of a solution, you may wish to control the pH, during EDTA complexation titrations (see p, 152) or preparative experiments involving carbonyl compounds, and one of the most effective ways to control pH is to use a buffer solution. 

EDTA is a remarkable reagent not only because it forms chelates with all cations except alkali metals but also because most of these chelates are sufficiently stable for titrations. This great stability undoubtedly results from the several com-plexing sites within the molecule that give rise to a cage-like structure, in which the cation is effectively surrounded by and isolated from solvent molecules. One of the common structures for metal/EDTA complexes is shown in Figure 17-3. The ability of EDTA to complex metals is responsible for its widespread use as a preservative in foods and in biological samples, as discussed in Feature 17-4. 

I irai Spreadsheet Summary The titration curve for the titration of Ca " I "I with EDTA is derived by both a stoichiometric approach and a master equation approach in Chapter 9 of Applications of Microsoft Excel in Analytical Chemistry. The effect of pH on the shape and end point of the titration curve is examined. 

Figure 4. Minimum pH for effective titration of various metals by EDTA (reproduced with permission from reference 12 Copyright 1958 American Chemical Society).

Minimum pH for effective titration of various metal ions with EDTA. (Reprinted with permission from C. N. Reilley and R. W. Schmid, Anal. Chem., 30 (1958)... 

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